It was first found to be of the closed form by Schwinger [1]}, [2]}, [3]}. (See also [4]} for the deduction from the duality between the 2D Ising model and the Ponzano-Regge model). It is in a form of a scaleless function:
\(\begin{split}{\mathcal {S}}(\lbrace z_l,\tilde{z}_l\rbrace )\equiv {\mathcal {S}}^{\text{sl}}_{\text{tet}}(\lbrace z_l,\tilde{z}_l\rbrace )=G(\lbrace z_l,\tilde{z}_l\rbrace )^{-2}\,,\quad G(\lbrace z_l,\tilde{z}_l\rbrace )&=1+\sum _{{\mathcal {L}}}\prod _{v\subset {\mathcal {L}}\,,\, l \prec l^{\prime } } [z_l|z_{l^{\prime }}\rangle \\&=1+\sum _{(3c)}^4\prod _{\begin{array}{c}v\subset (3c) \\ l \prec l^{\prime } \end{array}}\left[ z_{l}|z_{l^{\prime }} \right>+\sum _{(4c)}^3\prod _{\begin{array}{c}v\subset (4c) \\ l \prec l^{\prime } \end{array}}\left[ z_{l}|z_{l^{\prime }} \right>\,,\end{split}\) 
